원숭이도 나무에서 떨어질 때가 있다

This proverb (잠언) means, there are times when even monkey falls from tree. In other words, even an expert can make mistakes sometimes.

So, it can be used in various situations either to indicate sarcasm or when we want to encourage someone.

When a person who is said to be an Expert makes a mistake (e.g.) a Marketing Automation Expert makes a mistake while creating an Automation Campaign
Or for encouragement (e.g.) When there is a new person in team, and that person makes a mistake while doing something, then to encourage that team member

원숭이도 : Even Monkey (Monkey too*) | 도 : even/too

나무 : Tree

에서 : From e.g. 나무에서,

떨어지다 : Falling

ㄹ/을 때가 있다 : There are times when

그녀를 보고싶을 때가 있더라.

그분도 일할때 실수 해떤게 있더라.